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1 Advances in Environmental Biology, 5(10): , 2011 ISSN This is a refereed journal and all articles are professionally screened and reviewed ORIGINAL ARTICLE The Divergence Field in Western Iran Heavy Rain Associated with Central Mediterranean Cyclone Elham Mubarak Hassan and Katayon Varshosaz Department of Agriculture and Natural Resource, Ahvaz branch, Islamic Azad University, Ahvaz, Iran. Elham Mubarak Hassan and Katayon Varshosaz; The Divergence Field in Western Iran Heavy Rain Associated with Central Mediterranean Cyclone Abstract: In this study, the divergence filed were examined in three Mediterranean cyclone, producing heavy rain in Western Iran. It can found to be a common feature in surface, divergence filed and upper-level pattern for produce heavy rain. The analyses of these cases also indicate structure divergence filed favorable for the heavy rainfall in western Iran event included 1) The upper-level divergence prior to the development of heavy rain close to Iraq and western Iran and provided the upper-level support needed for the development of heavy precipitation, 2) located upper-level divergence in left side subtropical jet stream (STJ) or dominate the exit of jet. When the exit of subtropical jet with indirect circulation approach close to Iraq and Iran and maximum wind speed will be about 40 to 50 m/s, associated with divergence in the left side or dominate exit, rising branches in the exit regions of the subtropical jet that contributes to a forming of upward motion and produces clouds and precipitation in exit of jet. Therefore, the western Iran heavy rain was related to an upper-level trough associate with STJ in based on, propagating to Western Iran while surface cyclone is located in Turkey with extended pressure trough to Iran and Saudi Arabia. In the other words, the temporally increasing upper-level mass divergence and upward vertical motion near the exit region of the STJ is coincided with the development of the heavy rain area. Key words: divergence, subtropical jet stream, Mediterranean cyclone. Introduction In the cold season some of the cyclones cross the Mediterranean sea toward eastern Mediterranean. These depressions approach from west to Iran or cyclone is located eastern Mediterranean and pressure trough extending to western Iran. Also during the winter, the area of western Iran is under the influence of the southwestern edge of the Siberian anticyclone. Thus, pressure gradient is increased over Western Iran associated with heavy rain events. The idealized model of upper-level divergence, associated with a straight upper-level jet streak, shows that jet streak induces transverse circulation in entrance and exit of the jet. The upper-level divergence is strongly related to surface cyclogenesis and middle troposphere vertical motion and heavy rain [10]. The patterns of divergence associated with upper-level jet stream and the direct and indirect circulation have also been related to the development of severe convection and heavy rain in is considered in different parts of the world by many studies [13,8,2,11,7]. Horn and Achtor and Horn [1] show that development of storms occurs within an area marked by diverging longitudinal ageostrophic wind fields downstream of a trough axis and diverging transverse ageostrophic components within the exit region of the upper-level jet streak across the Western United States. The upper level divergence centers are expected to be found near upper-level jet stream upon the precipitation center. Although, Sometimes Heavy snowfalls are found between the exit and entrance regions of the two separate jet streaks. In a study of 18 intense snowstorms along the East Coast of the United States, Kocin and Uccellini [4], show that in general, heavy snowfall occurs to the north and northwest of the coastal surface low, downwind of an upper-level trough nearing the East Coast characterized by the divergence of the height contours(or diffluence), and within the exit region of an upper-level jet streak. Nakamura [6] analyzed the 10 years of National Meteorology Center data of the winter Asian jet and found that the convergence center to the left of the jet entrance dominates the entrance region, whereas the divergence pattern to the left of the exit dominates the exit region. Li et al. [3] shows that the upper-level divergence was observed prior to the development of heavy precipitation over northwestern Taiwan. The upper-level divergence behind a trough is a unique Corresponding Author Elham Mubarak Hassan, Department of Agriculture and Natural Resource, Ahvaz branch, Islamic Azad University, Ahvaz, Iran. Mobarak_e@yahoo.com

2 feature for heavy rainfall event and provides an important upper-level support for the development of heavy precipitation over Taiwan. In an analysis of several cyclones in the northern Pacific, Sinclair and Elsberry [12] conclude that the entrance and exit regions of jet streaks are important for providing the environment in which these particular storms can grow rapidly. Convergence in the lower and divergence in the upper troposphere, resulted in intense ascending motion over the area, enhanced thunder activity on eastern Mediterranean [7]. The investigation to daily extreme precipitation events for the winter season during 5 years demonstrated the divergence side of the jet stream coincides with the area of interest and supports enhanced ascending motions that is one of the necessary conditions for precipitation [11]. The purposes of this paper are 1) To present a synoptic patterns of Mediterranean cyclone in eastern Mediterranean with heavy rain, 2) to focus on the subtropical jet streaks with divergence fields and its role in the development of the heavy rain area in western Iran. In section 2, the initial data and methodology is described. Brief synoptic analyses and descriptions of divergence filed, jet stream situation and vertical circulation to provide evidence that the interaction of jet streak circulation patterns with three heavy snow events along the Western Iran are discussed in section 3. The results of the study are summarized in section four. Data and methodology: The NCEP-National Center for Atmospheric Research (NCAR) reanalysis was used for the analysis of synoptic, upper-level wind divergence fields and jet stream. The reanalysis is an objective gridded of historical data for the years 1948-present. The data 3426 grid resolution is and referred to the mean sea level and isobaric levels of 1000, 925, 850, 700, 500, 400, 300, 250, 200, 150, 100 hpa available every 6 hours. The domain considered was from 10 to 50 latitude and from 10 to 60 longitudes, which covers most of middle Mediterranean sea to western Iran. The area between 27.5 To 40 N and 37.5 to 50 E is marked by bold square and show changing pattern near the Iran prior to rain events. The rainfall data achieved to the rain gauge in synoptic stations. The total rain exceeding more than 30 mm is criterion for heavy rain in western Iran stations. In this way, three heavy rain events along the western Iran are recognized presenting by table 1. The total rain is occurred about 755 mm on 17 March 1993, 657 mm on 09 Dec 1991 and 494 on 08 March 1993, respectively. Sardashat station with N and 45.5 E geographical location has received accumulations rain in exceeding between of 91 mm to 132 mm in all of three cases. In each cases first, mean sea level pressure pattern and convective rain ( then, at 300 hpa level, geopotential pattern wind speed as isotach ( )and divergence (10 ),10 ignore in the text, will be analyze. Also, The vertical motions ( ) will be displayed in 500 hpa level. In addition, more to describe, the divergence field and vertical motion meridional cross section will be presented later. Result: In following, upper-level feature will be analyzed for finding familiarity between heavy rain events in three selected cases. In this section, a brief survey of the upper-level feature including upperlevel jet streams, upper level divergence, upward motion be analyzed and synoptic evolution will be analyzed during 12 hours prior to maximum rain events in western border Iran. Table 1: Synoptic station and total rain in three cases. 8-Mar Dec Mar Station 8-Mar Dec Mar Station Bijar Oroomieh Broujerd Ravansar 71 7 Eslamabad Saghez Ilam Sannandaj Kangavar Sardashat Khoramabad Tabriz Mahabad Takab Maragheh Zarineh Mianeh a) 17 March 1998: At 12(UTC) 16 March 1998, the cyclone with minimum central pressure contour 995 hpa is located eastern Mediterranean sea and southern Turkey from which pressure trough is extended to the Red sea in 12 UTC 16 March(Fig.1a). In addition, showing by Figure.1a, the convective rain more than 2 cm value

3 has covered Turkey to northwestern Iran located northeast cyclone, maximum upward motion Magnitude in 500 hpa is located northeast cyclone too exactly coincided with convective rain region. It may be recognized that upward motion contributed to produce heavy rain events but far away from Iran border, showing by marked square. At 300 hpa analysis, a low height is in Balkan Peninsula whose height trough extends from Northern black sea to central Mediterranean sea(fig. 1b). The cyclone is centered between trough and ridge line, in 12 UTC 16 March, that is consistent with the basic pattern associated with trough-ridge systems as described by Bjerknes and Holmboe. The Figure.1b depicts maximum wind speed 60,a well known subtropical jet stream hereafter called STJ, is centered at the base of trough over North Africa coast also elongated to north Saudi Arabia in the way that the exit of STJ covers Iraq and western Iran. The separated jet streak magnitude 30 is seen over the north of black sea nearby the ridge line in 12 UTC 16 March (Fig. 1b). At 300 hpa, divergence region exceeds 2 is developed some where between the STJ left exit and northern jet streak entrance close to western Turkey and ridge line. Comparing Fig.1a and b,it can be found, the upper-level divergence exactly is above convective rain and upward vertical motion area; besides, the surface cyclone is located beneath the diffluent exit region (left side) of the STJ in 12 UTC 16 March. The plan of vertical cross section constructed in 45 E, near the western Iran, is constructed to illustrate the transverse circulations, the vertical motion and divergence in the exit regions of STJ. The meridional cross section indicates maximum wind speed 60 near the 200 hpa between 25 to 30N, and second maximum wind speed (jet streak) 30 about 300 hpa in 50N ;in addition, demonstrates divergence 2.5 value, at 300 hpa in 40 N, in the other hand maximum convergence center -1.5 in the low level about 600 hpa from 37.5 to 45 N that is exactly located beneath the upper-level divergence which is between STJ and northern jet streak (Fig. 2a). The cross section of upward vertical motion reveals maximum 0.4 about 35 to 45 N at 400 hpa level between upper-level divergence and low-level convergence which provides by upward wind direction arrows (Fig. 2b). Vertical circulations must be supported at upper and lower levels by divergent horizontal motion. Since the geostrophic wind is nondivergent on a pressure surface, ageostrophic flow contains the divergent portion of the flow, which contributes to the vertical circulation patterns. The occurrence of precipitation suggests that the rising branches of indirect transverse circulation associated with STJ and direct transverse in entrance north jet streak could be contribute to the ascending 3427 motion needed for development of heavy rain. So the convective rain and upward vertical motion are placed beneath the left exit of STJ, which is accompanied with maximum upper-level divergence region. Upper-level pattern changing during the next 12 hours will be analyzed in following. The cyclone is weakened to 1000 hpa as remained in the same region,while pressure trough is extend toward northern Saudi Arabia during 12 hours in 00 UTC 17 March (Fig 1c). In the other hand, Siberian pressure ridge is progressed to south west of Iran so gradient pressure improves near the western Iran. The convective rain is moved to western Iran while enhanced to 4 cm which is accompanied with increasing maximum vertical motion to -0.5 in east of pressure trough located in the marked square (Fig. 2c). During 12 hour ending to 00 UTC 17 March, the upper-level patterns was changed so that the low height is weakened but stays in the previous positions while more obvious ridge line,also the maximum wind speed exceeds to 70 m/s with 10 m/s increase in base of low height. The STJ exit is getting close to western Iran while northern jet streak is further from STJ (Fig. 2d). The Figure 1d depicts decreased divergence to 1.5,in 300 hpa level, while shifted to south nearing the western Iran dominating exit STJ exit as located exactly over the upward motion and convective rain in 00 UTC 17 March, square marked. The Figure 2c represent evolution of meridional cross section in which magnitude upper-level divergence 2.5 is located about 200 hpa moved to south from 40 N to 32.5 N during before12 hours ending at 00 UTC 17 March ;in besides, convergence developed close to surface level near 900 to 800 hpa beneath upper-level divergence whrerby upward motions increased to -0.6 and transported to 600 to 500 hpa level in western border Iran. Although upper-level divergence is weakened during 12 hours, upward vertical motion is intensified, may be it is due to increasing low-level convergence and extended vertical divergence filed. Comparing Figure. 2b and d, It can be figure out the wind speed increased and wind shear intensified in lower surface above 30 to 35 N and also spread vertical of divergence filed, whereas the wind speed STJ is 60 the same as 12 hours ago. The meridional cross section reveals indirect circulation especially upward wind arrow emphasizing upward motion in the STJ left exit, in 00 UTC 17 March. In general, the cross section represented horizontal and vertical components wind of indirect circulations that contributed to temperature and moisture advection patterns in the low levels, it can also represents a widespread region of ascending motion that would be accompany to development of heavy rain in the western border Iran.

4 3428 Fig. 1: a) mean sea level pressure (5 hpa), hatched area as convective rain(1 cm) and dashed line as upward motion in 500 hpa every 0.1 interval ; b) thin line as geopotential height( ) every 80 interval, thick line as isotach ( ) every 10 interval and hatched area as divergence (10 ) :at 300 hpa 12(UTC) 16 March ; c) same as a and d) same as b but 00 (UTC) 17 March, 1998 a) c) b) d) Fig. 2: The cross section shown in Fig 1 by thick line ;a)bold line as isotach (wind speed) every 10 and dotted as divergence filed 10 every 0.5 ;b) dotted as upward motion every 0.1 and wind arrow in 45 E at 12 (UTC) 16 March ; c) same as a and d) same as b but at 00 (UTC) 17 March; 1998

5 b) 09 Dec 1991: Presenting by Figure. 3-a, in 12 (UTC) 08 Dec, the 1000 hpa cyclone is located western Turkey while 1010 hpa extended pressure trough toward north of Saudi Arabia, Also appearing in Fig. 3a, the maximum convective rain yielded 2 cm is occurred south of Turkey nearby to east of cyclone that exactly coinciding with upward motion 0.35 in 500 hpa in 12 UTC 08 Dec. At 300 hpa, extending trough line from western Russia to northeast of Mediterranean sea (Balkan Peninsula) in west edge surface cyclone is pronounced by Figure.3b while maximum wind speed with 50 is recognized as STJ, which elongated to Iraq and western Iran, with cyclonic curvature along northern east coast of Africa is in based on trough in 12 (UTC) 08 Dec. Besides, the Figure.3b depicts maximum divergence 1.5 between trough and ridge line,where covering east of Mediterranean to black sea, located left exit side of STJ and right entrance side of northern jet streak situated north of black sea. Comparing Figure 3.a and b, it can be found, the maximum upward motion with value 0.3 in 500 hpa level developed beneath upper-level divergence where maximum rain events is produced near the surface. To illustrate the transverse circulation patterns associated with the STJ and divergence filed nearby the western Iran in 45 E, meridional cross sections were constructed normal to jet axis. It displays the maximum divergence more than 2 is formed between 200 to 100 hpa above the maximum wind speed with value about 50 m/s while convergence near the surface has magnitude 0.1 that is less than of upper-level divergence magnitude located about 30 to 35 N, as a result weakly upward motion 0.1 is formed about 900 to 700 hpa level from 30 to 35 N above the western Iran in 12 (UTC) 08 Dec. It can be found that the vertical spread divergence field and upward motion are weak and no rain takes place in western Iran in 12 UTC 08 Dec. The meridional cross section is represented in 45 E, while in the 35 E maximum upward motion and upper-level divergence located aloft the heavy are far from Iran(not shown by cross section). In the next 12-hours, the all the structures is changed specially close to Iran in marked square. The cyclone is weakened to 1005 hpa in 00 UTC 09 Dec although it is located in the early situation, while pressure trough is closed to south of Iraq and north east of Saudi Arabia, exactly consequently inside the square (Fig.3c). In the other hand, Siberia pressure ridge is propagated to south west of Iran causing increasing pressure gradient. The heavy rain is increased to 4 and is shifted to the western Iran as the maximum upward motion is increased to 0.4,which is coincided with heavy rain, located east of pressure trough in 00 (UTC) 09 Dec By the upper-level feature, 300 hpa level, it can be found the low is increased 10 (showing in square) but divergence is decreased to 1 and shifted to exit of STJ, in addition northern jet streak is getting further from STJ (Fig. 3d). Thus, as exit region STJ associated with height and ridge line are situated in early situation while intensified wind speed along the north of Saudi Arab decreasing divergence filed approach to western Iran. Analyzing Figure. 3c and d shows that upper-level divergence, upward motion and heavy rain are coincided in vertical plan nearby Iran The Comparison of the meridional cross section in Figure.4a and b in 45 E, demonstrates upper-level divergence value 2 at 200 to 100 hpa above 30 to 40 N as convergence increase to 2.5 beneath near the surface in 32.5 N, so extended divergence filed in vertical plan is due to spread wind speed to down level and intensified vertical wind shear. The result of all processes is increasing upward motion to -0.5 between 30 to 35 N, western Iran, about 700 hpa level, in 12 hours early. The wind arrows present indirect circulation in exit of STJ whereas the rising branches of the indirect circulations were marked by ascent wind arrow about 32 to 40 N (Fig. 4d). Consequently, moveing eastward maximum wind speed(subtropical jet),the exit of STJ with divergence filed approach to western Iran, in other hand convergence is also increased nearby the surface and upward motion improved in mid-level favorable for producing precipitation. The construction of a cross section required the indirect circulations presents that may it transport humidity to mid-level and provide sufficient humidity so the rain value increase to 4 in western Iran. c) 08 March 1993: In the third case the heavy rain was occurred in 08 March In this case, The cyclone with central minimum pressure 995 hpa placed over the Turkey with 1005 hpa extended pressure trough to Saudi Arabia can be seen by Figure. 5a in 12 (UTC) 07 March. The convective rain area spread over Turkey to north west of Iran, although the maximum values 2 is took place in eastern Turkey coinciding with (Fig. 5a) the maximum upward magnitude 0.3 in 500 hpa located east of cyclone 12 (UTC) 07 March(Fig. 5a). At the upper-level, 300 hpa, the low height is recognized in Balkan Peninsula and ridge line nearby the northern Black sea (Fig. 5b). The STJ is elongated in north coast Africa (25 to 30 N) to Persian gulf with maximum wind speed 70, Which is located in south of low height. The divergence more than 1 10 is dominated in exit of jet stream close to downstream ridge line, seeing above heavy rain area and upward motion by comparison Fig. 5a and b in 12 (UTC) 07 March.

6 3430 Fig. 3: a) mean sea level pressure (5 hpa), hatched area as convective rain(1 cm) and dashed line as upward motion in 500 hpa every 0.1 interval ; b) thin line as geopotential height( ) every 80 interval, thick line as isotach ( ) every 10 interval and hatched area as divergence (10 ) :at 300 hpa 12(UTC) 08 Dec ; c) same as a and d) same as b but 00 (UTC) 09 Dec, a) b) c) d) Fig. 4: The cross section shown in Fig 3 by thick line ;a)bold line as isotach (wind speed) every 10 and dotted as divergence filed 10 every 0.5 ;b) dotted as upward motion every 0.1 and wind arrow in 45 E at 12 (UTC) 08 Dec ; c) same as a and d) same as b but at 00 (UTC) 09 Dec; 1991.

7 3431 Fig. 5: a) mean sea level pressure (5 hpa), hatched area as convective rain(1 cm) and dashed line as upward motion in 500 hpa every 0.1 interval ; b) thin line as geopotential height( ) every 80 interval, thick line as isotach ( ) every 10 interval and hatched area as divergence (10 ) :at 300 hpa 12 (UTC) 07 March ; c) same as a and d) same as b but 00 (UTC) 08 March, a) b) c) d) Fig. 6: The cross section shown in Fig 5 by thick line ;a)bold line as isotach (wind speed) every 10 and dotted as divergence filed 10 every 0.5 ;b) dotted as upward motion every 0.1 and wind arrow in 45 E at 12 (UTC) 07 March ; c) same as a and d) same as b but at 00 (UTC) 08 March; Analyzing meridional cross section feature in west of Iran(45E) indicates maximum wind speed 60 between 400 to 150 hpa accompanied with the upper-level divergence value in left side STJ over the 35 to 40 N (Fig. 6a). The convergence near surface which is located beneath

8 the upper divergence has 1 10 value more than upper level divergence value in 30 to 35 N, hence causing upward motion 0.3 at 700 hpa level in 35 N (Fig. 6a and b). During 12 hour ending 00 UTC 08 March, the increased pressure cyclone to 1000 hpa with extended pressure trough to north west of Iran is remained in earlier situation whereas direction turn to west-east in 08 (UTC) 08 March (Fig. 5c). Increasing gradient pressure is caused by propagating pressure ridge (Siberia) to southwestern Iran. The upper-level geopotential feature is maintained same the 12 earlier in 300 hpa(fig. 5d). Convective rain 3 cm value is took place west of Iran increased to previous 12 hour shifted from Turkey (Fig. 5c). Also the maximum upward motion is increased to 0.5 not exactly coincided with rain area (Fig. 5c). Changing isotach pattern is seen during 12 hours ending to 00(UTC) 08 March,decreasing maximum wind speed to 65 over the north Africa while the cyclonic curvature intensified in STJ exit close to western Iran in 00 (UTC) 08 March(Fig. 5d); Besides, the increasing divergence to 2 10 is appeared in exit of STJ in, showing by compare Fig. 5b and d. The vertical cross section displays maximum wind speed 60 between 300 to 150 hpa in 30 N; in addition, intensification divergence filed to is happened in left side STJ while the surface convergence is increased to in western Iran in 00 UTC 08 March(Fig. 6c). Besides, divergence filed is spread in vertical plan accompanied with intensification vertical shear wind between 30 to 35 N hence vertical motion is increased to 0.6 about 500 hpa level between upper-level divergence and low-level convergence(fig.6d). It can be recognized by Fig. 6d intensified indirect circulation in exit of STJ specially upward branch associated with maximum upward motion. As a result when STJ exit approach to north of Saudi Arabia and west of Iran, the wind speed and upper-level divergence are intensified over the western Iran beneath STJ exit, that is accompanied with increasing low-level convergence and upward motion, so it can be transport humidity to mid-level providing suitable conditions for occurrence of precipitation. Conclusions: An examination of three heavy rain events along the western Iran has shown common synoptic feature in surface and upper-level for produce heavy rain. The analyses of these cases also indicate structure divergence filed favorable for the heavy rainfall in western Iran included 1) The upper-level divergence prior to the development of heavy rain close to Iraq and western Iran and provided the upper-level support needed for the development of heavy 3432 precipitation,2) located upper-level divergence in left side STJ or dominate the exit of jet, 3) increased upper-level divergence between low height over Balkan Peninsula and ridge line over northern black sea, 4) intensified the surface convergence due to increasing upper-level divergence and 5)intensified upward motion about 0.4 to 0.6 between upper-level divergence and surface convergence. In the other words, the temporally increasing upperlevel mass divergence and upward vertical motion near the exit region of the STJ coincided with the development of the heavy rain area The upper-level divergence associated with STJ reveals a particular configuration in three cases. In the upper-level it can be found the upper-level STJ axis is centered between 27.5 to 30 N, extending from north of Africa coast to Saudi Arabia and Persian Gulf. The maximum wind speed in core STJ is about 50 to 70 associated with divergence filed in left exit STJ when the exit of STJ approach close to Iraq and Iran maximum wind speed will be about 40 to 50 m/s. The general similarities of this cross section constructed for the three cases appear to indicate processes associated with the circulation pattern in the rising branches of the transverse circulations in the exit regions of the STJ contributing to a region of ascent producing clouds and precipitation beneath the exit of STJ. Therefore the occurrence of western Iran heavy rain is related to existence STJ in based on upper-level trough which is propagated to Western Iran located above surface cyclone with extending pressure trough to western Iran and North Arabia Saudi. References 1. Achtor, T.H. and L.H. Horn, Spring season Colorado cyclones. Part I: Use of composites to relate upper and lower tropospheric wind fields, J. ClimateAppl. Meteor., 25, Hakim, G.J. and L.W. Uccellini, Diagnosing Coupled Jet-Streak Circulations for a Northern Plains Snow band from the Operational Nested-Grid Model, Wea. & Fore, 7: Li, J., Y.L. Chen and W.C. Lee, Analysis of a heavy rainfall event during TAMEX. Mon, Wea. Rev., 125: Kocin, P.J. and L.W. Uccellini and R.A. Petersen, Rapid evolution of a jet streak circulation in a re-convective environment. Meteor, Atmos. Phys., 35: and, A survey of major East Coast snowstorms, Part 2: Case studies of eighteen storms. NASA TM 86196, 214 pp. [NTIS N

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